The invention relates to a raster scanning system and, more particularly, to a scanning system utilizing a rotating polygon whose facets are uniformly illuminated in an overfilled design.
Many prior art raster output scanners (ROS) utilize a rotating polygon having flat reflective surfaces, or facets, in parallel with the axis of rotation of the polygon. In a typical system, a beam is emitted from a light source such as a helium-neon laser. The light is directed through a pre-polygon conditioning optics, modulated according to an input electrical signal, onto the rotating polygon surfaces. The high speed rotation of the polygon, typically in 3 to 15 krpm range, then scans the beam through a post-polygon conditioning lens and images the laser spot across the full process width of a photosensitive image plane. In these prior art ROS systems, the pre-polygon conditioning optics typically are incorporated in an underfilled facet design; e.g. the light beam directed against the rotating polygon illuminates only a portion of each rotating surface (facet). Overfilled facet designs, where the light beam completely illuminates each facet and a small portion of adjacent facets, have been used to some degree, but have not gained wide acceptance. Comparing the two designs, in an overfilled design the facet size required to produce a given spot size at the image plane is greatly reduced allowing many more facets to be accommodated on the same diameter polygon. This, in turn, permits the scan system to operate it at relatively low rotation rate permitting the use of less powerful (and less expensive) polygon motor drives. This advantage has, heretofore, been more than offset, by two factors: low throughput efficiency and non-uniform illumination at the polygon facets. In order to tolerate the low efficiency (typically 10 to 15%) , a higher powered laser diode is required. The non-uniformity problem is illustrated with reference to FIG. 1. FIG. 1 shows an illumination profile in an overfilled facet design. A beam 2, derived from a laser source, has a Gaussian spot shape which has been expanded so that more than one facet 4 of rotating polygon 6 is illuminated. As the polygon 6 rotates in the indicated direction to scan the spot across an output medium (not shown), the amount of light reflected to the medium varies because the facets are sampling different parts of the Gaussian illumination profile and the effective area of the polygon is changing.
When comparing the advantages and disadvantages of the two polygon configurations, the underfilled design has been preferred because of the relatively high throughput efficiency (50%). However, the lower rotation rate of the overfilled design makes it a desirable alternative if the uniformity problem can be resolved. According to the present invention, a pre-polygon optical system, including an aspherical lens system, provides for uniform spot illumination of a polygon facet operating in an overfilled mode. It is known to produce a uniform flat intensity profile from a Gaussian input using two spherical lenses. U.S. Pat. No. 4,492,435, FIG. 3, shows a lens system 58 which produces a collimated light beam which is supplied to a total internal reflection type of modulator. This type of lens system is unsuitable, however, for producing uniform profile inputs to a polygon surface because of high aberration (up to 10 wavelengths).
According to another aspect of this invention, the spot illumination at each facet of a rotating polygon is increased at the edges to compensate for illumination fall-off due to the effective size change at the polygon surface as the beam hits it head on at center of scan and at an angle at the ends of the scan. More particularly, the invention is directed towards a raster input scanning system utilizing an overfill polygon facet design for forming line images at a photosensitive medium comprising:
means for providing a beam of radiant energy, PA1 means for modulating said beam energy in response to electrical signals, PA1 means for expanding the modulated beam, PA1 a polygon having at least one reflective facet positioned in the optical path of said modulated beam and adapted to scan said spot across said medium, and PA1 an aspheric lens system positioned between said modulating means and said polygon, said aspheric lens adapted to transform the modulated output beam into a beam having a generally flat uniform intensity profile which overfills said facet as it rotates therethrough.